As the foreclosure crisis continues to infect Bay Area residential neighborhoods, county governments are facing one ripple effect. Rats and mosquitos are moving in as homeowners move out.

The latest figures from RealtyTrac, a housing data company, offer more grim news about the housing crisis for the region. In November, there was a 56 percent increase in foreclosure notices sent out to homeowners compared to last year, and more than 6,000 homes are in one stage of the three-part foreclosure process. That means more abandoned houses and neglected properties, which invite a host of unwanted visitors, according to county officials.

Swimming pools, which once offered warm-weather family fun, now are a public health hazard as breeding grounds for mosquitos, carriers of the West Nile virus.

"We have one of the higher foreclosure rates in the state or the nation here in San Joaquin," said Aaron Devencenzi, of the San Joaquin Mosquito & Vector Control District. "At any one time, we're watching from 100 to 2,000 neglected swimming pools on neglected properties."

Devencenzi said his office has had to spend more of its budget to hire extra seasonal workers to monitor the pools at foreclosed properties. When they get calls from neighbors complaining about abandoned pools, workers arrive to introduce mosquito fish, which eat the larvae and prevent a population explosion. "We explain that the pool will still look nasty," he said. "It isn't going to look clean and blue."

Getting access to foreclosed houses can be difficult, so Devencenzi has been doing outreach to realtor associations, asking them to call the district if the properties they're selling have pools that need attention. "Most of this is in urban areas and can be hard to find, so we put ads in the paper and let people know they can call us," he said.